Preamp ideas for F5

[buzzforb]

As shocking as this may sound Tinitus, i know the cap would accomplish this task, but as I said above, i know caps have audible effects, so not using them is ideal. I apologize if my trying to learn something is cumbersome to your reading. You do have the option of not reading. It is possible that others may have similar questions but are afraid to speak up due to more experienced members tendency to point out deficiencies in thinking. If i have worn out my time in this thread I will happily bow out and learn by smoking, as I have in most cases.

[Zen Mod]

catchin' Jaccogrumpitis ?

[tinitus]

wow..I just thought others, like some newbies, it might be worth to notice and remember a cap, just in case..

but ouch...sorry about that

[buzzforb]

Originally Posted by Zen Mod

catchin' Jaccogrumpitis ?

oh hush Hey you still havent told me why my wide baffle is bad idea. Are you avoiding me. Do we have a problem

Sorry Tinitus. Its that time of the month.

[Zen Mod]

Jaccobuzzin

I'm havin' that time of month ........ for entire month

nuff with grumpin' da thread , will send you results and comments later tonight

[juma]

Originally Posted by buzzforb

...every time i plugged it into main amp, offset whet crazy...

Something is obviously wrong because if you have 0mV at the preamp's output and 0mV at amp's input nothing should happen when connecting these two nodes. Retrace your steps and find the culprit.

Originally Posted by buzzforb

how do you match for tempco...

Empirically. I know that high gm JFETs have zero tempco at Id somewhere around 70-90% of Idss and I set the Id in that region with 20R trim-pot in the JFET's source. Next thing to do is to fiddle that pot until you find the value where Id stays the same no matter if JFET is cold or hot (cold means you blow on it to cool it and hot means you touch it gently with soldering iron's tip).
If you have means to measure the gm/transfer curve of the JFET you'll find out that JFETs with matching curves have matching zero tempco points too.
For practical purposes, Idss matching, thermal coupling and and using a trim-pot in the circuit are sufficient.

Anyway, we all know that friends are there to have their benevolence abused from time to time but it would be nice to surprise them pleasantly once in a while by doing the homework, so a bit thicker skin and cheering up is all we need

[buzzforb]

Originally Posted by juma

Something is obviously wrong because if you have 0mV at the preamp's output and 0mV at amp's input nothing should happen when connecting these two nodes. Retrace your steps and find the culprit.

If I had to guess, I would think it was may ground layout. Board layouts are a significant learning curve all by themselves.

Originally Posted by juma

Empirically. I know that high gm JFETs have zero tempco at Id somewhere around 70-90% of Idss and I set the Id in that region with 20R trim-pot in the JFET's source. Next thing to do is to fiddle that pot until you find the value where Id stays the same no matter if JFET is cold or hot (cold means you blow on it to cool it and hot means you touch it gently with soldering iron's tip).
If you have means to measure the gm/transfer curve of the JFET you'll find out that JFETs with matching curves have matching zero tempco points too.
For practical purposes, Idss matching, thermal coupling and and using a trim-pot in the circuit are sufficient.

I have a curve tracer that is o the blink, at the moment. Ill play around with it and see what I come up with. I used Rs of 10 on both source legs, with 200ohm pot in parallel, so i should be in same area as the 20omh pot you suggested. For me, the unknown is the BJT. I cut my short teeth on Jfets and Mosfets ad BJT's just seem like an alien in terms of operation.

Originally Posted by juma

Anyway, we all know that friends are there to have their benevolence abused from time to time but it would be nice to surprise them pleasantly once in a while by doing the homework, so a bit thicker skin and cheering up is all we need

[Rixsta]

hey is B1 ok matched with F5, or do you think i would need a Pre-amp
Sources, CD, Computer etc, i seem to remember the F5 not needing much more amplification, B1 is "class-a" right ? just need confirmation

[AndrewT]

If you want to take advantage of the peak voltage levels available from the F5 then you need a source that can push out ~2.6Vac (~3.5Vpk)
Very few, if any, domestic sources can achieve that.

[billyk]

The B1 "buffer" being unity gain, may work just fine with those sources, others may need more gain, that's what being discussed here. Personally I will be using this or something like it for a bit more gain than the B1 has to offer.